This invention relates to an ultrasonic transducer having a laminate structure, an array type ultrasonic probe formed by linearly arranging the transducers and a production method thereof.
The array type ultrasonic probe according to the prior art has the construction in which transducer elements made of a piezoelectric ceramic material are arranged in an array on a backing material, and a matching layer for establishing matching of acoustic impedance of an object or an acoustic lens is arranged in front of the transducers. The ultrasonic probe uses only one layer of the oscillation element or a laminate structure of a plurality of layers of oscillation elements (refer to "IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control", Vol. 43, No. 4, July 1996, pp620-622). In either of these layered structures of the prior art, electrodes opposing each other while interposing the piezelectric material are so formed for each layer as to extend substantially in the same width from one end to the other in a longitudinal direction of the transducer element. When a pulse voltage is applied to such an electrode, the oscillation element undergoes thickness oscillation and generates ultrasonic waves. The sound pressure level of this ultrasonic wave exhibits a rectangular distribution that is substantially uniform from one end to the other of the transducer element.
However, considerably strong side lobes appear on both sides of a main beam in the ultrasonic beams emitted from the transducer having such a sound pressure distribution, as represented by broken lines in FIG. 3, as is known in the art. The signals generated from these side lobes mix as the noise with the signal obtained from the main lobe and reduce a signal-to-noise ratio (S/N) of the signal from the main lobe.
To reduce such detrimental side lobes, it is known to progressively reduce the ultrasound pressure level from the center towards both end portions. This means is described in "IEEE, 1985, Ultrasonics Symposium", 1985, pp793-799.